Method of tapping aluminum from a cell for electrolytic recovery of aluminum

ABSTRACT

The metal height to be tapped is determined by using the amount of ampere-hours and two positions of the anode beam. Corrections are made for changing of the thickness of the lateral ledges of frozen electrolyte.

United States Patent Chaudhuri Aug. 12, 1975 METHOD OF TAPPING ALUMINUM FROM A CELL FOR ELECTROLYTIC RECOVERY [56] References Cited 0F ALUMINUM UNITED STATES PATENTS [75] inventor: Kiranendu B. Chaudhuri, Gampel, 3,660,256 5/1972 Lippitt et al. 204/67 swtzerland FOREIGN PATENTS OR APPLICATIONS Assigneei Swiss Aluminium PP 187,318 3/1965 U.S.S.R 204/67 Switzerland [22] Filed: Jan. 24, 1974 Primary Examiner-John H. Mack Assistant ExaminerAaron Weisstuch [21] Appl 436l61 Attorney, Agent, or FirmEmest F. Marmorek [30] Foreign Application Priority Data [57] ABSTRACT Aug. 9, 1973 Switzerland 11498/73 The metal to be tapped is determined using 1 the amount of ampere-hours and two positions of the 204/245 anode beam. Corrections are made for changing of the [5 Int. CL thickness of the lateral ledges of frozen electrolyte [58] Field of Search 204/67, 225, 245, 243 R,

3 Claims, 2 Drawing Figures PATENTED AUG 1 21975 3, 899 .402

Figi Fig. 2

mm. mm,

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METHOD OF TAPPING ALUMINUM FROM A CELL FOR ELECTROLYTIC RECOVERY OF ALUMINUM BACKGROUND OF THE INVENTION For the recovery of aluminum by electrolysis of alu minum oxide (A1 alumina) the latter is dissolved in a fluoride melt, which consists in the greatest part of cryolite Na AlF This melt is contained in a cell, the inner walls of which consist of amorphous carbon. Anodes of amorphous carbon dip from above into the melt. The aluminum separated at the cathode collects in liquid state on the bottom of the cell beneath the fluoride melt. Oxygen is released at the anodes by the electrolytic decomposition of the aluminum oxide, and combines with the carbon of the anodes to CO and C0 The electrolysis takes place in a temperature range of about 940 to 975C.

The principle of an aluminum electrolysis cell with prebaked anodes appears from FIG. 1, which shows a schematic vertical section in the longitudinal direction through part of an electrolysis cell. The steel shell 12, which is lined with a thermal insulation 13 of heatresisting, heat-insulating material, e.g. chamotte, and with carbon 11, contains the fluoride melt (the electrolyte). The aluminum l4 separated at the cathode lies on the carbon bottom 15 of the cell. The surface 16 of the liquid aluminum constitutes the cathode. In the carbon lining 11 there are inserted iron cathode bars 17 (in this case transverse to the longitudinal direction of the cell), which conduct the electrical direct current from the carbon lining 11 of the cell laterally outwards. Anodes 18 of amorphous carbon dip. from above into the fluoride melt l0, and supply the direct current to the electrolyte. They are firmly connected via conductor rods 19 and by clamps 20 with the anode beam 21. The anode beam can consist of one or more conducting bars.

The current flows from the cathode bars 17 of one cell to the anode beam 21 of the following cell through conventional bus bars, not shown. From the anode beam 21 it flows through the conductor rods 19, the anodes 18, the electrolyte 10, the liquid aluminum 14, and the carbon lining 11 to the cathode bars 17. The electrolyte 10 is covered with a crust 22 of solidified melt and a layer of aluminum oxide 23 lying above it. Cavities 25 occur in operation between electrolyte l0 and the solidified crust 22. Against the side walls of the carbon lining 1] there likewise forms a crust of solid electrolyte in the form of the lateral ledge 24. The thickness of the ledge 24 determines the horizontal ex tent of the bath of fluid aluminum l4 and electrolyte 10. With rising temperature, the thickness of the ledge 24 generally decreases, with falling temperature generally increases.

The average distance d from the lower sides 26 of the anodes to the upper surface 16 of the liquid aluminum, which is also known as the interpolar distance, can be adjusted by lifting or lowering of the anode beam 21 with the help of the lifting mechanismus 27, which are mounted on pillars 28. This operates on all the anodes. Each anode can however be adjusted by raising or lowering singly, if the respective clamp 20 is opened, the conductor rod 19 is shifted relatively to the anode beam 21 and finally the clamp 20 is again closed. Because of the attack by the oxygen released during'electrolysis, the anodes are consumed continuously on their lower face by about 1.5 to 2 cms per day (anode burning) according to the type of cell, and simultaneously the level of the liquid aluminum rises by about the same amount because of the separation of aluminum at the cathode.

When an anode is used up, it must be exchanged for a new one. The cell is so operated in practice that, some days after starting up, the anodes of the cell no longer have the same degree of consumption and therefore after use for several weeks they must be exchanged separately. For this reason one finds anodes of different starting age operating together, as appears from FIG. 1.

The horizontal surface, which contains the totality of the lower faces of the anodes of a cell, is known as the anode table.

The principle of an aluminum electrolysis cell with self-baking anodes (Soederberg anodes) is the same as that of an aluminum electrolysis cell with pre-baked anodes.

Instead of pre-baked'anodes, anodes are used which, during the electrolytic operation, are continually baked from a green electrode paste in a steel jacket by the heat of the cell. The direct current is supplied by lateral steel rods or from above by vertical steel rods. These anodes are renewed as required by pouring green electrode paste into the steel jacket.

By breaking in the upper electrolyte crust 22 (the crusted bath surface) the aluminum oxide 23 which is above it is brought into the electrolyte 10. This operation is known as servicing of the cell. In the course of the electrolysis the electrolyte becomes depleted in aluminum oxide. At a lower concentration of for example 1 to 2.5% of aluminum oxide inthe electrolyte, there arises the anode effect, which results in a sudden increase in voltage from the normal 4 to 4.5 volts for example to 20 volts and above. Then at the latest the crust must be broken in and the Al O concentration be raised by addition of new aluminum oxide.

In normal operation the cell is usually serviced periodically, even if no anode effect occurs. This cell servicing will be referred to in what follows as normal cell servicing. It occurs for example every two to six hours. In addition, as stated above, upon every anode effect the crust of the bath must be broken in and the A1 0 concentration raised by addition of fresh Al O which corresponds to a cell service. Thus in operation the anode effect is always associated with a cell service, which, in contrast to normal cell service, can be referred to as anode effect service.

The aluminum 14 produced electrolytically, which collects on the carbon bottom of the cell, is generally tapped once a day from the cell, eg by conventional sucking devices. Generally the level of the liquid aluminum 14 is brought back to an optimum value for each type of cell. This value corresponds to the desired metal level, which can be the starting level.

An important characteristic value in the operation of a cell is its electrical base voltage. This is established empirically for each cell having regard to its age, the condition of the carbon lining 11, the composition of the electrolyte melt 10 as well as the cell current intensity and current density. For the establishment of the base voltage regard is also had to the horizontal extent of the cathode surface 16, which is influenced by the thickness of the lateral ledge 24.

From the base voltage the base resistance of the cell can be calculated according to the following equation:

R is the ohmic base resistance in ohms, U the base voltage in volts, 1.65 the back electromotive force in volts and J the instantaneous cell current intensity in amps.

For the actual voltage to equal the base voltage, the

interpolar distance must have an optimum value. If the cell is so operated that the horizontal extent of the cathode surface 16 remains unchanged, then generally the rise in level of the liquid aluminum above the carbon bottom is equal to the burning away of the anodes at their lower face. Thecell is designed so that these conditions are reached. If then in these circumstances one wants to tap the metal to an extent which will return the metal level to a starting level, then it is sufficient just to tap a height of liquid metal that corresponds to the burning away of the anodes. In practice the actual interpolar distance is from time to time, e.g. between two tapping operations, larger or smaller than the optimum interpolar distance. The departures are substantially caused by irregular rise in the level of the liquid aluminum above the carbon bottom, by irregular burning away of the anodes at their lower face, and by variation in the horizontal extent of the cathode surface 16 as a consequence of alteration of the thickness of the lateral ledge 24. If in this circumstance one taps the metal in a cell exactly by the amount which corresponds to the burning away of the anodes, then one does not reach the desired metal level in the cell, but the cell is over or under tapped, that is to say one has tapped too much or too little metal.

My invention relates to a method of tapping aluminum from a cell for electrolytic recovery of aluminum in accordance with automatic determination of the metal height to be tapped.

SUMMARY OF THE INVENTION The method according to my invention allows the metal height to be tapped to be accurately determined, having regard to the varying thickness of the lateral ledge, the burning away of the anodes, the varying interpolar distance and the desired metal level, and the cell thereupon to be tapped to the desired metal level.

The method according to my invention for tapping aluminum from a cell for recovery of aluminum by electrolysis of aluminum oxide dissolved in a fluoride melt comprises the following operationaLsteps carried out in succession:

a. at regular time intervals the instantaneous ohmic cell resistance is calculated, the instantaneous values over a certain period of time are smoothed and the difference AR between this smoothed cell resistance and the base resistance established for each cell is calcu lated;

b. as soon as to 60 minutes after a normal cell servicing the difference AR exceeds a limiting value given for each cell, the anode beam is raised or lowered, in order to match the existing ohmic resistance with the ohmic base resistance,

0. the difference AB of the vertical levels of the anode beam is calculated from two values, of which the first is taken 30 to minutes after the normal cell service following the previous tapping operation, and the second is taken 30 to 60 minutes after the last normal cell service before the next tapping operation referred to at (e) below;

d. the metal height H (mm) to be tapped is calculated according to the equation in which J signifies the mean direct current in ki loamps, r the time in hours which has passed between successive tapping operations, and f a proportionality factor kA h DESCRIPTION OF THE PREFERRED EMBODIMENT Below an advantageous example of a method according to my invention is described.

By a computer the cell voltage U and the cell direct current intensity J are sampled at regular time intervals, e.g. every 10 to 60 seconds, and the cell resistance is calculated from this according to the equation:

U 1.65 m: J

R is the instantaneous ohmic resistance in ohms, U is the instantaneous cell voltage in volts, 1.65 the back electromotive force in volts and J the cell direct current intensity in amperes.

Simultaneously, e.g. with the help of a potentiometer arranged on the anode beam, the level of the anode beam of the cell is read off by the computer. J, R and the value of the level of the anode beam are stored in the computer. The values for R calculated by the computer are smoothed over a predetermined period of time, e.g. 10 minutes, and are compared at regular time intervals, e.g. every 10 to 15 minutes, with the base resistance R,, of the cell. If the computer notices a difference AR between the smoothed value and R and if this difference exceeds a limiting value previously given to the computer and stored in it, then an order is issued by the computer, in accordance with which the anode beam is raised or lowered, until the instantaneous resistance is substantially equal to the base resistance of the cell. This adjustment is carried out 30 to 60 minutes after a normal cell service. The values of the level of the anode beam before and after each movement are read by the computer, for instance by means of the potentiometer, and are stored in the computer.

the cell is calcu- H =1, t .f+ AB in which H is the metal height to be tapped from the cell in millimetres, J the mean direct current of the cell in kiloamps, t the time in hours which elapses between two successive tapping operations, f a proportionality factor. The dimensions of f are mm/kA.h, and it undertakes the conversion of kiloamp hours into millimetres of anode burning away. Usually fis in the range 0.0056 to 0.0063. AB in millimetres is a difference between two levels of the anode beam.

To calculate the difference AB of the levels of the anode beam, reference is made to the following two values of level. The first value is taken 30 to 60 minutes after the normal service following the previous tapping operation. The second value is taken 30 to 60 minutes after the last normal service before the next tapping operation. The times at which the two values of level are taken need not be equally far in time from the respective normal service. From the two values the difference AB is obtained.

BRIEF DESCRIPTION OF FIG. 2

FIG. 2 illustrates in a diagram the times at which the level of the anode beam is taken. 50 is the instant of a previous tapping operation, 51 the instant of the first normal cell service following it. At the instant 52, which lies 30 to 60 minutes after the first normal service 51, the first value of the level of the anode beam is taken. At 53 occurs the next normal tapping operation. At the instant 54 occurs the last normal service before the tapping operation 53. At the instant 55, which lies 30 to 60 minutes after the normal cell service 54, the second value of the level of the anode beam is taken. Between the instants 52 and 54 there can lie further normal cell services or anode effect services (not indicated in FIG. 2).

CONTINUATION OF THE DESCRIPTION OF THE PREFERRED EMBODIMENT It is to be noted that, during the time range 30 to 60 minutes after a normal service, the actual resistance (R in ohms) of the cell established by adjustment of the anode beam should not depart from the base resistance (R in ohms) by more than about i l X 0. This restriction is necessary, because if the limiting value of: l X 10 is exceeded, the departure of the actual interpolar distance from the optimum interpolar distance ceases to be negligible. If the limiting value is not exceeded at each instant of taking the level of the anode beam, then one can reckon that the two measurements of the level have been undertaken at substantially equal interpolar distance. The taking of the levels of the anode beam at the mentioned determined times after a normal service on the cell is important, because during this time the alumina concentration in a cell reaches its maximum value. During this time the influence of alumina concentration on the cell voltage is practically negligible.

6 The-tenn-AB in the formula H J,', t'.f+ AB has its origin ir'ft he'results'of possible variation in the thickness of the lateral ledge of the cell. If the difference AB is equal to zerogth'en one can conclude that there isno alteration in the ledge thickness. All'other values of AB indicate an'alteration of the ledge thickness. The presence of AB in the formula means that in a long run, that is over a length-of time covering several successive tapping operations, the level of the aluminum is repeatedly returned to an optimum level, and hence the thickness 'of the lateral ledge cannot undergo progressive change, but only minor irregular changesaround a mean value.

If, during the first taking of the level of the anode beam, an anode effect occurs, or manipulations are carried outon the cell, which disturb the taking of the level (c.g. erroneously carried out movements of the anode beam), this value of the level should not be employed for obtaining the difference AB. In this circumstance either AB is arbitrarily set equal to zero, or the provision of the level of the anode beam is again undertaken 30 to 60 minutes after the next following normal service.

If during the second taking (55) of level of the anode beam an anode effect occurs, or manipulations are carried out on the cell, which disturb the taking of the level, this value also should not be made use of in forming the difference.

AB in this case is again arbitrarily set as nil, or the level of the anode beam 30 to 60 minutes after the previous normal service is provided from data stored in the computer.

When the value of the height of metal to be tapped has been calculated according to the invention, then the tapping is carried out.

The accuracy of the tapping depends on the devices available. In order to increase the accuracy of tapping, the device, a conventional sucking device for example, can be controlled by a computer. Then the computer starts the tapping, observes the metal level by simultaneous lowering of the anode beam so as to keep the ohmic resistance of the cell constant, and interrupts the tapping operation as soon as the metal level has been reduced by the given height.

The advantages of the method according to the invention lie in the fact that the metal height to be tapped is automatically determined and that this determination gives more accurate results in comparison with existing technique. If one always taps the metal height calculated according to the invention, a uniform tapping in ensured, and thus an excessive or insufficient tapping from the cell is avoided. Thus a uniform cell operation is achieved, which leads to improvement of the current efficiency and of the specific electrical energy consumption.

What I claim is:

l. A method of tapping aluminum from a cell for recovery of aluminum by electrolysis of aluminum oxide dissolved in a fluoride melt, comprising the following operational steps:

a. at regular time intervals the instantaneous ohmic cell resistance is calculated, the instantaneous values over a certain period of time are smoothed and the difference AR between this smoothed cell resistance and the base resistance established for each cell is calculated;

b. as soon as 30 to 60 minutes after a normal cell servicing, whenever the difi'erence AR exceeds a limitservice following the previous tapping operation, and the second is taken 30 to 60 minutes after the last normal cell service before the next tapping operation referred to at (e) below;

d. the metal height H (mm) to be tapped is calculated according to the equation in which J signifies the mean direct current in kiloamps, t the time in hours which has passed between 8 successive tapping operations, and f a proportionality factor (mm/kA h) e. tapping is carried out to reduce the metal level by the height given by the equation under (d).

2. A method according to claim 1, in which use is made of a computer, which controls a sucking device, starts the tapping, observes the metal level by simultaneous lowering of the anode beam so as to keep the ohmic resistance of the cell constant, and interrupts the tapping operation as soon as the metal level has been reduced by the given height.

3. A method according to claim 2, in which the levels of the anode beam are determined with means comprising a potentiometer arranged on the anode beam. 

1. A METHOD OF TAPPING ALUMINUM FROM A CELL FOR RECOVERY OF ALUMINUM BY ELECTROLYSIS OF ALUMINUM OXIDE DISSOLVED IN A FLUORIDE MELT, COMPRISING THE FOLLOWING OPERATIONAL STEPS: A. AT REGULAR TIME INTERVALS THE INSTANTANEOUS OHMIC CELL RESISTANCE IS CALCULATED, THE INSTANTANEOUS VALUES OVER A CERTAIN PERIOD OF TIME ARE SMOOTHED AND THE DIFFERENCE $R BETWEEN THE SMOOTHED CELL ESISTANCE AND THE BADE RESISTANCE ESTABLISHED FOR EACH CELL IS CALCULATED, B. AS SOON AS 30 TO 60 MINUTES AFTER A NORMAL CELL SERVING, WHENEVER THE DIFFERENCE $R EXCEEDS A LIMITING VALVE GIVEN FOR EACH CELL, THE ANODE BEAM IS RAISED OR LOWERED, IN ORDER TO MATCH THE EXISTING OHMIC RESISTANCE WITH THE OHMIC BASE RESISTANCE, C. THE DIFFERENCE $B OF THE VERTICAL LEVELS OF THE ANODE BEAM IS CALCULATED FROM TWO VALUES, OF WHICH THE FIRST IS TAKEN 30 TO 60 MINUTES AFTER THE NORMAL CELL SERVICE FOLLOWING
 2. A method according to claim 1, in which use is made of a computer, which controls a sucking device, starts the tapping, observes the metal level by simultaneous lowering of the anode beam so as to keep the ohmic resistance of the cell constant, and interrupts the tapping operation as soon as the metal level has been reduced by the given height.
 3. A method according to claim 2, in which the levels of the anode beam are determined with means comprising a potentiometer arranged on the anode beam. 